Shim6 vs PI addressing

Iljitsch van Beijnum iljitsch at
Wed Mar 1 18:08:59 UTC 2006

On 1-mrt-2006, at 18:05, David Barak wrote:

> Is it easier to scale N routers, or scale 10000*N hosts?

Is it easier for the government to make a 5 year plan or for everyone  
to spend time and energy finding the best deal for everything?

Every router has to search through its FIB tables for every packet it  
forwards. That's something like 10 FIB lookups for every packet  
flowing between two hosts. The hosts only have to search through  
their TCBs for every packet. Number of TCBs in nearly all hosts is  
smaller than the average FIB size (even if you consider that many  
routers don't have a full table). 2 x relatively small is a lot less  
than 10 x relatively large. Or, in other words: on the host you only  
pay if you actually communicate. In routers, you pay more as there is  
more routing information, whether the extra information is used or not.

> If we simply moved to an "everyone with an ASN
> gets a /32" model, we'd have about 30,000 /32s.  It
> would be a really long time before we had as many
> routes in the table as we do today, let alone the
> umpteen-bazillion routes which scare everyone so
> badly.

1. We've already walked the edge of the cliff several times (CIDR had  
to be implemented in a big hurry, later flap dampening and prefix  
length filtering were needed)
2. We'll have to live with IPv6 a long time
3. Route processing and FIB lookups scale worse than linear
4. If the global routing table meltdown happens, it will be extremely  
costly in a short time
5. Even if the meltdown doesn't happen a smaller routing table makes  
everything cheaper and gives us more implementation options (5000  
entry TCAM is nice, 500000 entries not so much as it basically uses  
100 times as much power)
6. Moore can't go on forever, there are physical limitations

But the most important thing we should remember is that currently,  
routing table growth is artificially limited by relatively strict  
requirements for getting a /24 or larger. With IPv6 this goes away,  
and we don't know how many people will want to multihome then.

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